One of the largest studies ever conducted of dementia in moderate-income countries has found the incidence similar to that in more affluent Western countries.

For years, health experts have believed that dementia -- the degenerative loss of memory and other brain functions -- was less common in middle-income countries than in wealthier, developed countries. But when researchers from Kings College, London conducted a 10-year survey of six developing countries -- Cuba, China, the Dominican Republic, Mexico, Peru and Venezuela -- they found the incidence of dementia was roughly the same as in developed countries.

Martin Prince, who directs the Global Center for Mental Health at King's College, says data from the study suggest that, in fact, most cases of dementia are occurring in the developing world.

"We think that two-thirds of all people with dementia in the world live in the middle-income countries at the moment. But the treatment gap is actually huge, so at least 90 percent get no diagnosis," Prince stated. "No treatment and no care whatsoever."

There are approximately 36 million people with dementia worldwide. Prince says the figure doubles every 20 years. As his study suggests, most of the people who will be diagnosed with dementia in the next 40 years will be in developing countries.

In the United States, dementia is defined by precise guidelines outlined in a text called the Diagnostic and Statistical Manual of Mental Disorders, or DSM. But many experts believe those guidelines miss many mild and moderate cases of dementia. So Prince and his colleague used a more sensitive dementia measure to interview nearly 12,800 individuals aged 65 and older in middle income countries, comparing their findings to the DSM guidelines.

Prince says individuals suspected of having dementia and their family members were interviewed extensively. Investigators found that the incidence of dementia is between 1.5 to five times higher than would have been the case using DSM guidelines.

The study by Prince and colleagues also found, as in Western countries, the level of educational attainment and mental stimulation among older individuals seems to delay or ward off the effects of mental decline.

"So essentially people with richly developed, really well-functioning brains can afford to lose a bit of nerve cells and nerve tissue, and have damage to that network and show really not many, or no discernable signs, of cognitive problems," said Prince.

Prince says identifying older people with mild to moderate dementia in middle income countries prepares families for their inevitable decline, hopefully leading to improved care.

An article by Martin Prince and colleagues on dementia in middle income countries is published in the journal The Lancet.